This invention relates to a process for generation of acid and for imaging, and to an imaging medium for use in this imaging process.
Some conventional non-silver halide photosensitive compositions, for example photoresists, contain molecules which are inherently photosensitive, so that absorption of a single photon brings about decomposition of only the single molecule which absorbs the photon. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the photosensitive molecule initiates a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each photon absorbed. For example, photoresist systems are known in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction. See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; Reichmanis et al., Chemical Amplification Mechanism for Microlithography, Chem. Mater., 3(3), 394 (1991) and Berry et al., Chemically Amplified Resists for I-line and G-line Applications, SPIE, 1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes a radiation-sensitive mixture which contains a water-insoluble binder which comprises a mixture of phenolic and novolak polymers and which is soluble or dispersible in aqueous alkali, and an organic compound whose solubility in alkaline developer is increased by acid, and which also contains at least one acid-cleavable group, and in addition a further group which produces a strong acid upon exposure to radiation.
U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixture using a monomeric silylenol ether, and a recording medium produced therefrom. This patent also contains an extensive discussion of radiation-sensitive compositions which form or eliminate an acid on irradiation. According to this patent, such radiation-sensitive compositions include diazonium, phosphonium, sulfonium and iodonium salts, generally employed in the form of their organic solvent-soluble salts, usually as deposition products with complex acids such as tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonic acid and hexafluoroarsenic acid; halogen compounds, in particular triazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyrones which contain trichloromethyl or tribromomethyl groups; aromatic compounds which contain ring-bound halogen, preferably bromine; a combination of a thiazole with 2-benzoylmethylenenaphthol; a mixture of a trihalomethyl compound with N-phenylacridone; .alpha.-halocarboxamides; and tribromomethyl phenyl sulfones.
The aforementioned phosphonium, sulfonium and iodonium salts are superacid precursors which, upon exposure to ultraviolet radiation, decompose to produce superacids, that is to say acids with a pK.sub.a less than about 0. Other materials decompose to produce superacids in a similar manner. However, all the superacid precursors require ultraviolet to blue visible radiation for decomposition (see, for example, Crivello and Lam, Dye-Sensitized Photoinitiated Cationic Polymerization, J. Polymer Sci., 16, 2441 (1978)), and the need for this radiation is disadvantageous when it is desired to produce high resolution images, which are most conveniently produced by laser imaging. In the present state of technology, diode lasers emitting at near infra-red wavelengths of about 700 to 1200 nm. provide the highest output per unit cost. Near infra-red solid state lasers emitting at about 1000-1200 nm. are also useful in imaging processes, while ultraviolet lasers are costly. Accordingly, it is desirable to find some way in which superacid precursors can be rendered susceptible to infra-red radiation in order that imaging of a superacid precursor-containing medium can be effected using an infra-red laser.
It is already known that various sensitizing dyes can catalyze the decomposition of superacid precursors upon exposure to wavelengths to which the superacid precursors are not sensitive in the absence of the sensitizing dye. Unfortunately, due to the difficulty of protonating the superacid anion consequent upon the very low pK.sub.a of the superacid, the sensitizing dye is protonated by the superacid, so that no unbuffered superacid is produced in the medium (i.e., the sensitizing dye buffers the superacid produced). Since no unbuffered superacid is released into the medium, these processes cannot be used to trigger any secondary reaction which requires the presence of unbuffered strong acid, such as the reactions used in many photoresists, as described in the aforementioned patents.
(The term "unbuffered superacid" is used herein to refer to superacid which is not buffered by the sensitizing dye, and which thus provides an acidic species stronger than that provided by buffered superacid, that is to say superacid buffered by the sensitizing dye. Because of the extreme acidity of superacids and their consequent tendency to protonate even species which are not normally regarded as basic, it is possible, and indeed likely, that "unbuffered superacid" will in fact be present as a species buffered by some component of the imaging medium less basic than the sensitizing dye. However, such buffering by other species may be ignored for present purposes, so long as superacid is present as an acidic species stronger than that provided by superacid buffered by the sensitizing dye.)
This invention provides a process for generation of acid which enables a medium containing a superacid precursor and a sensitizing dye, which is more easily protonated than the superacid anion, to be imaged with radiation of a frequency to which the superacid precursor is not sensitive, so as to produce unbuffered superacid in the medium. By including an acid-sensitive material in the medium, the process can be used for imaging.